The bar-headed goose can reach nearly 21,120 feet, new study shows.

Editor's note: An October 2012 study in the Proceedings of the Royal Society Bfound that bar-headed geese follow valleys through the mountains, which keeps them below 18,000 feet nearly all of the time. However the birds do make occasional forays to higher altitudes, a feat that still can't be explained, according to the study.

The world's highest flying bird is an Asian goose that can fly up and over the Himalaya in only about eight hours, a new study finds.

The bar-headed goose is "very pretty, but I guess it doesn't look like a superathlete," said study co-author Lucy Hawkes, a biologist at Bangor University in the United Kingdom. (See bird pictures.)

In 2009, Hawkes and an international team of researchers tagged 25 bar-headed geese in India with GPS transmitters. Shortly thereafter, the birds left on their annual spring migration to Mongolia and surrounding areas to breed.

To get there, the geese have to fly over the Himalaya—the world's tallest mountain range and home to the tallest mountain on Earth, Mount Everest, which rises to 29,035 feet (8,850 meters).

The researchers that found the birds reached a peak height of nearly 21,120 feet (6,437 meters) during their travels. The migration took about two months and covered distances of up to 5,000 miles (8,000 kilometers).

The birds made frequent rest stops during the migration, but they appear to have flown over the Himalayan portion of their journey in a single effort that took about eight hours on average and that included little or no rest. A similar intense climb could kill a human without proper acclimatization, Hawkes said.

"If you've ever seen a goose sitting on a lake, take-off is quite an energetic thing, so it may be [energetically] cheaper to keep going than to keep sitting down and taking off again—and they may not want to delay getting over the mountains," Hawkes said.

Before the new study, many scientists had thought the geese were taking advantage of daytime winds that blow up and over mountaintops. But the team showed the birds forgo the winds and choose to fly at night, when conditions are relatively calmer.

"I guess it's like when you're trying to get in the grocery store and there's a steep set of stairs and a really long [wheelchair] route. You have to work out which one you can be bothered to do on a particular day," Hawkes said.

Another hypothesis about why the geese choose to fly over rather than around the Himalaya is that the birds have been doing so for millions of years—long before the mountains reached their current heights.

"Geese are a relatively old group of birds, and it's possible that when bar-heads first evolved as a species, the mountains weren't nearly as high as they are today," Hawkes said.